The present invention relates to a method and apparatus for burning high sulfur-high nitrogen fuels while reducing SO.sub.x and NO.sub.x pollutants.
A major problem which has arisen in the United States and other highly industrialized countries of the world is air pollution from exhaust gases from large stationary installations, such as boilers in power plants, large stationary gas turbines employed as a driving force in power plants and other large installations, as well as process heaters in refineries and other chemical operations. It is anticipated that this problem will almost certainly be aggravated in the relatively near future by the necessity of using lower quality fuels, such as fuels derived from heavy petroleum oils, shale oils, coal liquids, etc., as well as the direct combustion of solid fuels such as coals, lignites, etc. Such fuels contain relatively large amounts of fuel-nitrogen, e.g., up to about 2.5 percent or higher, as compared with presently available petroleum derived fuels containing very little, if any, fuel-nitrogen. For example, No. 4 and No. 6 petroleum oils contain about 0.1 to 0.5 wt. percent nitrogen. Thus, any crude oil containing less than about 0.5 percent nitrogen is considered a low nitrogen oil and the production of NO.sub.x pollutants in the flue gases from the burning of such oils is of little significance. However, heavy petroleum oils, crude shale oils and the like contain up to about 2.5 wt. percent or higher of chemically bound nitrogen and a typical crude solvent refined coal oil contains from about 1.0 to 1.5 wt. percent of chemically bound nitrogen, while a typical solid fuel, such as coal, lignite, etc., contains an average of about 1.4 wt. percent nitrogen. If all of the chemically bound nitrogen in the fuel is converted to nitrogen oxides, generally referred to as "fuel NO.sub.x ", 1 percent by wt. of nitrogen in a solvent refined coal oil has the potential of producing about 1.928 lbs/MM BTU or 1,300 PPMV (parts per million by volume at 3 percent excess oxygen, dry) of nitrogen oxides (NO.sub.x), while 1.85 and 1.93 percent by wt. of nitrogen in crude shale oils will potentially produce about 3.288 and 3.440 lbs/MM BTU (2,595 and 2,642 PPMV), respectively. In addition, nitrogen oxides produced by the "hot-air reactions" at flame temperatures, and referred to as "thermal NO.sub.x " also contribute to total NO.sub.x pollutants in the flue gases from combustion processes.
The federal limit for the discharge of NO.sub.x pollutants into the atmosphere from steam generators burning liquid fossil fuels (1974 EPA New Source Performance Standards [NSPS]) is 0.3 lbs/MM BTU (about 230 to 237 PPMV for typical shale oils). Some state limitations are even more stringent, for example the California standard is 225 PPMV. These limits include both fuel NO.sub.x and thermal NO.sub.x. While these standards can be met when burning low nitrogen (below about 0.1 percent), petroleum derived fuel oils, serious complications are encountered when high nitrogen fuels, such as heavy petroleum derived fuel oils, crude shale oil, crude coal oils and solid coals, lignites, etc. are burned in conventional utility boilers. For example, since thermal NO.sub.x increases with temperature, modern utility boilers, which preheat the combustion-supporting air to 600.degree. to 800.degree. F. for improved efficiency, produce thermal NO.sub.x alone which can approach the specified emission standards. Consequently, in order to meet these standards, the conversion of fuel nitrogen to NO.sub.x emissions, in a fuel having about 2.0 wt. percent bound nitrogen, should not be more than about 5 percent. It has been reported in the literature that, when shale oils with about 2.0 wt. percent nitrogen are burned in a stationary boiler of an electrical generating station, NO.sub.x emissions on the order of 700 to 900 PPMV can be anticipated and, when solvent refined coal oils, with slightly more than 1.0 wt. percent nitrogen are burned, at least 20 to 50 percent of the fuel nitrogen is converted to NO.sub.x emissions (260 to 650 PPMV).
While it has been suggested that high levels of fuel nitrogen can be reduced by blending the high nitrogen fuel with low nitrogen petroleum derived fuels or by burning crude high nitrogen fuels in selected burners of a boiler while burning low nitrogen petroleum derived fuel oils in other burners or the addition of additives to the fuel, the most promising technique to date has been a 2-stage, rich-lean combustion process, in which a primary combustion zone is operated fuel-rich and a secondary combustion zone is operated fuel-lean.
Another major source of air pollution is the production of SO.sub.x pollutants in the burning of fuels containing relatively large amounts of sulfur. For example, petroleum oils containing up to about 0.5 wt. percent of sulfur are considered low sulfur crudes or sweet crudes, whereas other petroleum oils contain up to about 2.5 wt. percent, or greater, of sulfur and are considered sour crudes. Generally, the sulfur content of petroleum oils, even though it is organic sulfur, can be eliminated by mild hydrotreating operations. On the other hand, the sulfur content of normally solid fuels, such as coals, lignites, etc. or liquids derived from shale, coal, lignite, etc. present an entirely different problem. Again, a coal having less than about 0.5 wt. percent sulfur is considered a low sulfur coal and can generally be utilized as such or cleaned up in a relatively simple manner to prevent air pollution. However, high sulfur coals contain anywhere from 0.5 to 8.0 percent by weight of sulfur in elemental or combined form. Sulfur in coal, depending upon the type, can be present as elemental sulfur, sulfate sulfur, mineral sulfur (ferrous disulfide-FeS.sub.2 usually referred to as pyritic sulfur) or organic sulfur, which is chemically bound. Sulfate sulfur is usually present in amounts less than about 0.1 percent by weight and is water soluble. Accordingly, this type of sulfur, along with elemental sulfur can usually be removed by water washing. On the other hand, pyritic sulfur is insoluble in water, but is heavier than coal and, therefore, can be removed by specific gravity separation, i.e., flotation, etc., or by relatively simple magnetic means. The amounts of pyritic sulfur vary widely with the nature of the coal, in some cases, constituting substantially all of the sulfur content of the coal, while in others, only about half of the sulfur of the coal. On the other hand, organic sulfur can constitute as much as half of the sulfur content of a given coal and is the most difficult to remove, since it requires destruction of the coal molecule itself.
Numerous solutions for the removal or reduction of SO.sub.x pollutants, due to burning a solid fuel, such as coal, lignite, etc., have been proposed. Physical cleaning of the coal is a relatively simple and commercially practiced technique and it involves the previously mentioned specific gravity separation techniques, magnetic techniques, etc. However, such techniques remove only pyritic sulfur, elemental sulfur and sulfate sulfur and are incapable of removing organic sulfur. Synthetic fuel production, such as the production of gaseous and liquid fuels from normally solid fuels is another proposed solution. However, in both instances, these processes are expensive, have not been commercially developed and, in addition, as previously indicated, synthetic liquids contain large amounts of nitrogen, which must be removed by some other means. Yet another proposed solution, which has been commercially developed and is utilized is desulfurization of the flue gas itself. While flue gases can be treated to remove both NO.sub.x and SO.sub.x pollutants and to do so within EPA and State limits, this is a highly expensive operation. Finally, the direct burning of solid fuels, such as coal, lignite, etc., has been proposed in fluidized bed combustion processes with additives, such as limestone, dolomite, etc. to remove sulfur. However, such combustion requires expensive equipment, is difficult to control and also requires substantial volumes of additives. A more promising approach to the desulfurization of solid fuels, such as coal, etc., is the chemical cleaning of coal. A wide variety of processes are involved, including mainly solvent partition, thermal decomposition, acid-base neutralization, reduction, oxidation and nucleophilic displacement. Unfortunately, none of these techniques has been developed to the commercial stage and, in general, most are considered relatively expensive. However, it is necessary to resort to such techniques in order to remove organic sulfur.
The biggest problem, associated with the reduction of pollution in the burning of fuels, is that most fuels which contain high concentrations of nitrogen also contains high concentrations of sulfur. It has been suggested that high levels of fuel nitrogen in petroleum oils can be reduced, along with the sulfur, by severe hydrotreating techniques, i.e., at extremely high pressures and high temperatures and utilizing large amounts of H.sub.2. However, such techniques do not remove sufficient nitrogen, they tend to crack the feed materials and such techniques have not been fully commercially developed. In addition, even if available, pilot plant tests indicate that such refining of crude shale oils and crude coal oils, would increase costs by about $3.00 to $5.00 per barrel. Unfortunately, researchers concerned with the removal of high levels of nitrogen from fuels have not given sufficient consideration to the removal of high concentrations of sulfur and in many cases have simply assumed that the equipment and conditions necessary for the removal of high concentrations of nitrogen will also remove high concentrations of sulfur. Similarly, researchers concerned with the removal of high concentrations of sulfur from fuels have also assumed that the same conditions and the same equipment will also be effective in the removal of sufficient nitrogen. Consequently, there is no proposed technique which will adequately and inexpensively remove high concentrations of both nitrogen and sulfur from fuels.